Phototube



D. G. HAKNES {2,112,089

PHOTOTUBE Filed Nov. 2, 1955 INVENTOR DONALD 6. HAINES Patented Mar. 22, 1938 ATENT oFFmE PHOTOTUBE Donald G. Haines, Newark, N. J., assignor, by

mesne assignments, to Radio Corporation of America, New York, N. Y., a. corporation of Delaware Application November 2, 1935, Serial No. 47,944

6 Claims.

My invention relates to light responsive devices and particularly to phototubes comprising a plurality of photo-cathodes.

It is an object of my invention to devise a phototube with a high current output response for small changes in light conditions.

A further object of my invention is a phototube which is independent of variations in light source intensities and particularly a phototube which will indicate the position and/or motion of a light beam.

It is still a further object of my'invention to provide an evacuated or gas filled phototube which in a balanced amplifier circuit will not decalibrate or unbalance the connected circuits because of uncontrolled changes in tube characteristics due, for example, to liberation of gases in the phototube.

To this end I provide a phototube in which a common envelope encloses two closely spaced photo-cathodes and an anode with the'electrodes so positioned as to prevent the activation of one cathode by a light beam directed .on the other. While the electrodes of my improved phototube are compactly arranged with the photo-cathodes facing or looking in the same direction, the cathodes are optically shielded one from the other. A phototube constructed in accordance with my invention may conveniently be coupled to a pushpull amplifier circuit with each anode-cathode path connected in diiferentbranches of the input circuit of the push-pull amplifier tubes. In such an adaptation .of my improved phototube, the amplifier circuits are found to be particularly stable, giving a strong output signal independent of light source intensities and independent of static changes in the phototube.

The novel features that I consider characteristic of my invention are set forthwith particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together With additional objects thereof, will best be understood from the following description of specific embodiments Figure 8 is a diagrammatic View illustrating one circuit application of my improved phototube.

For convenience of description, the side of my phototube from which light is received in normal operation is hereinafter referred to as the front of the phototube, while the base end of the phototube is called the bottom and the opposite end is designated as the top.

Referring to Figures 1, 2, and 3 of the drawing showing three views of a phototube embodying my invention, numeral I designates an evacuated envelope terminating at its lower end in a re-entrant stem with a press 2. The envelope may, if desired, be evacuated or filled with gas and may be constructed of a light pervious material, such as glass, mounted on any desired form of base. Embedded in the press are a plurality of upstanding support wires 3 and 4 which may serve as supporting means as well as lead-in wires for the electrodes.

One phototube constructed in accordance with my invention may comprise two semi-cylindrical photocathodes 5 and B mounted parallel and in closely adjacent upstanding positions within the envelope on the support wires 3, the photo-cathodesbeing held in spaced relation at their upper ends by a glass bead l and brackets 8. Each photo-cathode may comprise a metal framework, such as of nickel, with two elongated sheets or strips!) joined along their rear edgestosupportwire 2i and with curved fingers I0 extending outwardly and forwardly as best shown in Figures 1 and 3. To the finger portions may be secured a thin sheet of metal II, preferably of silver or silver plated nickel, with its edges folded over the ends of said fingers as at-IZ; Any desired light responsive material, such as caesium oxide deposited on oxidized silver, may be formed upon the concave surface of the metal sheets. Silver may conveniently be coated upon the concavesides of the cathodes-and sensitized by glowing in oxygen.

A single anode l3'may be employed in my improved phototube and may conveniently comprise a metallic ribbon supported parallel'to and forwardly of the adjacent edges of the photo-cathodes, as shown, upon rod 4 extending from the press and rod l4 extending from the glassbead 1. The plane of the anode is preferably normal to a plane through the forward edges of thephotdcathodes, so that only the forward or front edge of the anode is visible to a point in front of and equidistant fromthe photo-cathodes. Anode l3 in its position between the two cathodes eifec tively shields one photo-cathode fromlight diffused from the walls of the envelope in front of the other photo-cathode. As shown in Figure the outer edges of the semi-cylindrical photocathodes are, with respect to a point directly in front of the phototube, slightly to the rear of the adjacent edges of the photo-cathodes so that perpendicular bisectors of the major cords or planes through the forward edges of the photocathodes diverge outwardly.

The two parallel semi cylindrical photo cathodes are disposed with their light sensitive surfaces facing in substantially parallel directions, laterally of the tube. It is apparent that a beam of light directed perpendicular to the face of the drawing in Figure 2 can be made to fall selectively upon either photo-cathode 5 or 6 by a slight shift of the beam and that no rays of light directed upon either photo-cathode can illuminate and activate the other photo-cathode.

The top, front and side views of the phototube shown in Figures 4, 5, and 6 show a modified construction embodying the characteristic features of the phototube of Figures 1, 2, and 3. Here the support rods 3 extend longitudinally of the envelope with an offset at their lower end to position them near the rear wall of the envelope and may be held in spaced relation at their upper ends by glass bead 1. Each photo-cathode in this modification comprises an elongated metal sheet 20 transversely curved and joined along one edge to support rod 3'. The curved portion of each sheet extends forwardly and toward a vertical plane through the center of the tube. Each sheet may be of nickel and coated and sensitized with any desired light responsive substance upon the concave surface of the sheet, as in Figures 1, 2, and 3.

The anode 21 of the phototube of Figures 4, 5, and 6 may conveniently comprise a bent wire with two vertical sections or legs 22 joined at the top by a transverse section 23, which may conveniently be supported by and electrically connected to a cross-bar 24 and lead-in support wire 25 from the press. The two legs 22 may, if desired, be electrically separated and provided with individual lead-in connections. Each leg of the anode may be positioned at or near the center of curvature of its adjacent photo-cathode, as shown, the cathodes preferably being mounted upon their supports so that their centers of curvature are outside of any straight line drawn from the photo-cathode surfaces to a source of light in front of the phototube. The unobstructed exposure of the photo-cathodes to the front of the tube can best be seen in Figure 5.

In the arrangement of electrodes of Figures 4, 5, and 6, the photo-cathodes are in effect back-toback with the sensitized surfaces of each photocathode, being out of the optical range of the other and each being light shielded from a light beam directed upon the other. The anode is so arranged that the mean length of anode-cathode path between any one photo-cathode and its cooperating section of the anode is at a minimum, the anode sections being effectively at the focal point of the curved photo-cathodes.

Figure 7 shows a top view of a third modification of my phototube wherein the photo-cathodes 38] each comprise an elongated strip of nickel secured as in Figures 4, 5, and 6 along one edge to side rods 3 rising from the press. Here the strips are fiat and extend inwardly toward and parallel to a longitudinal line slightly displaced forward of the center line of the phototube. Each cathode may be activated by a light responsive coating on the diverging or forward (downward in Figure '7) side of the metal strips. The anode 3| in the embodiment shown in Figure 7 may be placed to the rear of the photo-cathodes and may conveniently comprise a single rod rising vertically out of the center of the press. It has been found that in such an arrangement of electrodes the photo-cathodes are effectively light shielded one from the other.

Figure 8 illustrates by way of example one circuit application of my improved phototube. Two amplifier tubes 40 and 4| connected in push-pull with the usual output transformer 42 are coupled at their inputs to a balanced bridge arrangement wherein resistances 43 and 44 form two legs of a bridge and the two anode-cathode paths of my phototube l comprise the other two legs of the bridge. Upon proper adjustment of resistance 44 the input impedances of the amplifiers may, with equal intensities of light upon the two sections of the phototube, be balanced. With a common envelope for the two sections of my phototube the bridge will not be unbalanced by changes in conditions in the phototube resulting, for example, from unstable gas pressures. Now with a light beam shifting from one photo-cathode to the other at a signal rate as, for example, from the mirror of a galvanometer, the frequency of shift may be indicated in the secondary of the output transformer as an alternating current of the frequency of the shifting light beam, the indication being independent of variations in light source intensities. It will be noted that in this circuit employing my improved phototube both extreme positions of the light beam serve to impulse the amplifier, thus doubling the amplitude of the output voltage. The amplifier may, of course, be provided with resistance coupling means in its input and output circuits to relay direct current indications of relative light intensities upon the two photo-cathodes.

While but three modifications of the preferred embodiment of my invention have been described, it will be obvious to those skilled in the art that many modifications may be made in my improved phototube without departing from the scope of my invention. It is accordingly desired that my invention be limited only by the prior art and the terms of the appended claims.

I claim:

1. A phototube comprising a light pervious envelope enclosing two parallel semi-cylindrical electrodes with their concave surfaces facing in substantially parallel directions and with the forward edges of the electrodes lying substantially in one plane and with adjacent edges parallel and closely spaced, an electron emitting light responsive coating on the concave surface of each of said electrodes, and an anode and light shield comprising a fiat metallic sheet disposed in a plane normal to said one plane and forwardly of said adjacent edges.

2. A phototube comprising an envelope enclosing two fiat sheet-like electrodes supported sideby-side and with the plane of one at an angle with the plane of the other, sensitized light-re sponsive coatings on the diverging surfaces of said electrodes, and a cooperating anode disposed rearwardly of the converging surfaces of said electrodes.

3. A phototube comprising a light pervious envelope enclosing two parallel spaced semi-cylindrical electrodes, a sensitized light responsive material on the concave surface of each electrode, the perpendicular bi-sectors of the concavities of the sensitized surfaces intersecting each other on a line on the convex side of said electrodes behind the sensitized surfaces, and electron collecting means disposed adjacent each bi-sector.

4. A phototube comprising an envelope with a reentrant stem, a support wire embedded in said stem, a photo-cathode comprising a strip of metal secured along one edge to said support wire with curved fingers outwardly extending from another edge of said strip, a metallic sheet with a light sensitive electron emitting coating shaped to the configuration of said fingers and secured thereto and an anode in said envelope.

5. A phototube comprising a light pervious envelope with a re-entrant stem supporting side by side two parallel sheet-like photo-cathodes sensitized with light responsive electron emitting material on one surface, and with adjacent edges closely spaced and with the perpendicular planes bisecting the light sensitive surfaces intersecting each other on a line behind the light sensitive surfaces, and electron collecting means in said envelope.

6. A phototube comprising a light pervious envelope enclosing two closely spaced sheet-like electrodes having vertical side edges, each electrode being sensitized on one surface with light responsive electron emitting coatings, the planes through said vertical side edges forming an angle on the non-sensitive side of the electrodes less than 180 so that the adjacent edges of said electrodes optically shield the sensitized surface of one electrode from the sensitized surface of theother electrode, and an electron collecting electrode mounted in said envelope.

DONALD G. HAINES. 

